1. Technical Field
The present invention relates to an insulating layer composition for a substrate, and a prepreg and a substrate using the same.
2. Description of the Related Art
With the development of electronic devices, a printed circuit board has continuously been requested to have a low weight, a thin thickness, and a small size day by day. In order to satisfy these requests, wirings of the printed circuit board becomes more complex and further densified. Therefore, electric, thermal, and mechanical stabilities of the substrate act as more important factors. In particular, a coefficient of thermal expansion (CTE) is one of the important factors determining reliability at the time of manufacturing the printed circuit board.
The printed circuit board is made of mainly copper for circuit wirings and polymer for interlayer insulation. The polymer constituting an insulating layer has a very high coefficient of thermal expansion, as compared with copper. In order to overcome this difference, there are mainly used materials in which a coefficient of thermal expansion of the polymer constituting the insulating layer is lowered by impregnating the polymer with a woven glass fiber or adding an inorganic filler to the polymer.
In general, as the added amount of inorganic filler gets increased, the coefficient of thermal expansion of the insulating layer becomes further lowered, but a process for manufacturing the printed circuit board has limitations in lowering it unlimitedly.
In addition, in order to satisfy a request for high density of fine patterns, a surface roughness of the insulating layer also has been important. Therefore, the size of the added inorganic filler gets smaller and smaller in order to secure this surface roughness. However, the size of the inorganic filler gets smaller, which causes problems in uniform dispersibility. Therefore, uniform dispersion of nano-scaled inorganic filler becomes a big problem.
FIG. 1 shows a structure of a printed circuit board, which is made of copper serving as a circuit wiring and polymer serving as an inter insulating layer. The copper (Cu) circuit wiring has a coefficient of thermal expansion of 10˜20 ppm/° C., and a normal polymer material used in the insulating layer has a coefficient of thermal expansion (CTE(a1)) of 50˜80 ppm/° C. Since the coefficient of thermal expansion of the polymer is largely increased at a glass transition temperature (Tg, 150 to 200° C.) or higher, a coefficient of thermal expansion (a2) at a high temperature of the polymer reaches 150 to 180 ppm/° C.
In addition, at the time when parts such as semiconductor devices are mounted on the PCB, heat is rapidly supplied to the PCB at about 280° C. for 3 to 5 seconds. Here, in case where a difference in the coefficient of thermal expansion between a circuit and an insulating layer is large, cracks may occur in the circuit or a substrate may be distorted.
Ultimately, a polymer material for an insulating layer, which has the same coefficient of thermal expansion as copper for the circuit wiring and a semiconductor chip to be mounted on the substrate, is needed. However, for now, it is difficult to meet the requirements for complex and highly densifying wirings of printed circuits by using a material obtained by regulating the kind of polymer constituting the existing insulating layer and the content thereof, and a size and a content of inorganic filler.
Meanwhile, there exist two types in polymer composite insulation materials used in the insulating layer for a printed circuit board. One type is a prepreg made by impregnating woven glass fabric (or woven glass cloth) with a polymer composite insulation material, followed by B-staging at a temperature of a glass transition temperature (Tg) of the material or lower, as shown in FIG. 2.
The other type is a film produced by using only the polymer composite insulation material not containing woven glass cloth, as shown in FIG. 3. According to the latter, a polymer composite insulation material, an inorganic filler, a hardener, a solvent, an additive, a hardening promoter, and the like are blended at an optimum blending ratio, followed by mixing and dispersing, and then post-cased in a film type.
A polymer composite insulation material for forming an insulating layer of the existing printed circuit board is mainly an epoxy resin. A CTE of the epoxy resin itself is approximately 70 to 100 ppm/° C. The epoxy resin is impregnated with woven glass fiber, or a large amount of inorganic filler having a small CTE is added into an epoxy matrix as shown in FIG. 4, thereby realizing a low CTE of epoxy resin.
The CTE value is mostly decreased in a linear type according to the added amount of filler. However, if a large amount of filler is added in order to lower the CTE, dispersibility of the inorganic filler within the matrix is largely degraded, and thus, aggregation of the filler may occur and surface roughness of the printed circuit board may be largely increased. Further, as viscosity of epoxy is easily increased rapidly, molding of products is difficult. In particular, in a case of a multilayer structure in, for example, as insulating films used in the printed circuit board, interlayer binding may be often impossible.
Due to this limitation, effects of the filler need to be promoted by lowering the CTE of epoxy resin itself and introducing a critical amount of inorganic filler ensuring lamination processability. For example, in order to lower the CTE of the epoxy resin itself, mainly, epoxy resins having different structures are mixed and used, and here, components of the respective resins and compositions thereof perform important roles.
Further, the kind, size, and shape as well as the added amount of the inorganic filler affect the CTE of the epoxy resin, and thus, miniaturization of the added inorganic filler, that is, a nano-scaled inorganic filler is requested in order to realize ultrafine patterns. However, even though the nano-scaled inorganic filler is added, there are still difficulties in obtaining a homogenous molding film through uniform filler dispersion.
Therefore, the development of materials for the insulating layer of the printed circuit board having a low CTE is needed.